Numerous techniques are known in the art for measuring the thickness of a transparent film by optical means. All require measuring the intensity of a light beam which interacts with the film, as some parameter of the system is scanned (e.g., wavelength, angle of incidence, polarization, etc.). Where it is necessary to monitor a changing film thickness, these techniques become impractically slow or expensive. Fortunately, the very thing which works against those techniques provides an alternative solution. The change in film thickness can itself be the necessary scanning variable.
It is a well known optical phenomenon that light of a given wavelength shining through or reflected from a transparent film will interfere with itself constructively or destructively, depending upon the thickness of the film. Therefore if a film is steadily increasing or decreasing in thickness, such interference will impose a sinusoidal intensity fluctuation on the transmitted or reflected light.
Cyclic changes in optical interference are well known in the prior art for monitoring changing thickness of a transparent film. In particular, it is well known to determine or control the absolute thickness of a deposited film, as in a sputter deposition process, by counting the number of cycles in the optical interference signal, starting when the film being deposited has zero thickness. The same technique has been applied to monitoring decreasing film thicknesses, but in this case the initial film thickness may not be known accurately enough to determine absolute thickness unambiguously.
This general technique has been improved by utilizing light of two slightly different wavelengths instead of one to form interference patterns, with the outputs of two detectors being subsequently combined by sum and difference amplifiers, for example, to more accurately identify the desired film thickness. With all such systems, however, a restriction existed that film thickness could only be analyzed with respect to a known reference or starting point. Cyclic repetition in the output of such systems provides only ambiguous information on absolute thickness, in the absence of other information.
On the other hand, in the semiconductor industry where it is necessary to etch away certain films such as quartz films, for example, the films are often of indeterminate thickness at the start of a processing step. For example, a usual etching process utilizes a silicon wafer having a quartz film deposited thereon which is to be etched away to a certain level. The thickness of the quartz film falls within a predetermined thickness range but the absolute thickness of the film will vary from wafer to wafer depending upon the accuracy of process control at an earlier stage in manufacturing. Accordingly, a need exists to know the absolute value of the thickness of quartz films utilized in semiconductor processing at some point prior to the completion of etching processing. This in turn would enable the prediction of a desired end point of the process so that the process end point could be reached without etching overrun.